The present invention relates to a semiconductor memory which functions as a static random access memory (RAM) or an electrically alterable read-only memory (EAROM).
In general, semiconductoor memories are classified into random access memories (referred to as RAMs hereinafter) and read-only memories (referred to as ROMs hereinafter). The RAM is a read/write memory in which data-write and data-readout can be freely achieved, while the ROM is a memory used only for data readout. The RAM and ROM are utilized in accordance with their functions. In a microcomputer, for example, a RAM is used for storing information that is often altered, if required, while a ROM is used for storing data such as a program which is not changed.
A RAM constituted by a semiconductor integrated circuit is essentially a volatile memory that cannot hold stored data when electric power is interrupted. On the other hand, the ROM is a nonvolatile memory that can hold stored data even if electric power is interrupted.
However, in addition to the conventional limited applications of the RAM and ROM, it is often required that stored data can be altered and made nonvolatile. For this purpose, ROMs have been developed which have the inherent nonvolatile property and can alter stored data if required. Of such ROMs, an EAROM (electrically alterable ROM) has an advantage in that stored data can be electrically erased without using an ultraviolet ray generator.
The EAROM utilizes, as memory cells, MNOS (Metal Nitride Oxide Semiconductor) transistors which have a silicon nitride film and a silicon oxide film between a gate electrode and a semiconductor substrate and can store electrons in the interface of both films. In case of a memory using MNOS transistors, write, readout, and erasure of data are performed by applying voltages to the gate electrodes of MNOS transistors. Therefore, data which has been written into MNOS transistors do not volatilize even if power is interrupted and can be read out after application of power.
As described above, the EAROM can operate in the same manner as the RAM. However, as compared with the RAM, the readout speed and especially the write speed are extremely slow. Further, in the EAROM, data stored in the memory cells cannot be selectively erased from cell to cell. Therefore, the EAROM cannot be essentially used as a RAM.